Gas discharge lamp with a collimating reflector electrode



B. A. STORY GAS DISCHARGE LAMP WITH A CQLLIMATING REFLECTOR ELECTRODEFiled June 2'7, 1961 2 Sheets-Sheet, 1

INVENTOR. BENJAMIN A. STORY I! H" w w:

ATTORNEYQ Patented Dec. 17, 1963 Filed June 27, 1961, Set. No. 120,969 3Claims. (Cl. 313-113) This invention relates to an electric dischargelamp and more particularly to a compact source lamp having an internalreflector.

Compact source lamps produce relatively high intensity illumination.However, the effective utilization of the illumination is generallydependent upon exterior reflectors. Such reflectors are used tocoll-imate and concentrate the light over a relatively small area.

These systems necessarily involve a loss of efliciency. For example asecond exterior reflector may be used to direct a portion of the lighttoward the collimating reflector. For many purposes these systems aresatisfactory however, in other cases it is desirable to overcome certainlimitations in order to obtain improved characteristics.

In systems capable of providing relatively high intensity illuminationsuch as sun simulator systems, it is desirable to direct a maximumintensity of light toward a collimating I'fiLSCtOT. in order to improvethe efliciency of such a system it is desirable to eliminate lightlosses caused by the light rays passing through the relatively thickquartz envelope. The use of a second exterior reflector causes multiplepasses by some of the light rays through the quartz envelope plusundesirable light scattering. In order to reduce these losses and todirect more light toward the collimating reflector, it is desirable toincrease the intensity output in a particular region of the lamp ,eventhough there may be a corresponding drop in total output.

In order to increase the intensity of the light in a preselectedquadrant of the lamp it is desirable to have an internal reflector. Theuse of such a reflector may decrease the overall efliciency of the lamp,however; the resulting increase in intensity in a preselected quadrantmore than compensates for the loss in total efliciency. The preselectedquadrant defines the portion of the lamp which directs high intensityillumination toward a collimating reflector.

Attempts have been made to coat the inner surface of the quartz envelopewith a reflective film. However, the heat in a lamp of this type isrelatively great and tends to destroy the reflective coating.

Advantageously a lamp structure according to the invention providesfiavorable performance characteristics. A relatively large percentage oflight is concentrated in in the quadrant facing the collimatingreflector. The total enregy output of the lamp may be decreased,however, the advantage of increasing the intensity in a preselectedquadrant more than compensates for any loss in the total energy output.

The reflectivity of tungsten is poor i.e. about 56% at 2800 centigrade.Also tungsten is a hard brittle material and hard to work. For thesereasons it would seem undesirable to incorporate a tungsten reflector.Notwithstanding these problems a smooth reflecting tungsten filament hasbeen incorporated in a novel compact source lamp according to thepresent invention. :It has been found possible to gather about 22% ofthe radiant energy with such a reflecting element and to reflect about/2 of this, or about 11% of the total energy, back through the electricarc. This results in increasing the energy output directed toward thecollimating reflector.

Briefly the present invention contemplates an electric discharge lamp ofthe type referred to as a short arc lamp. The lamp comprises a pair ofelectrodes arranged Within a transmissive envelope. The envelope isfilled with a rare gas or a mixture of inert gases such as argon,krypton and xenon either singly or in combination, and can also containa drop of mercury to provide mercury vapor on heating by the discharge.The pressure of the rare gas should lie above atmospheric pressure. Oneof the electrodes includes an integral reflector comprising a curvedconcave surface of revolution. This internal reflector increases theintensity of illumination in a preselected region of the lamp.

The invention will now be described in more detail in connection withthe accompanying drawings, in which:

FIG. 1 is a perspective view of a lamp according to a first embodimentof the invention;

FIG. 2 is a side elevational View in cross section of an anode and acathode according to the first embodiment of the invention;

FIG. 3 is a side elevational view in cross section illustrating a secondembodiment of the invention; and

FIG. 4 is a graphic representation of the polar distribution of thefirst embodiment of the invention compared with the polar distributionof a conventional lamp.

A compact source lamp according to the first embodiment of the presentinvention is illustarted in FIGS. 1 and 2. The lamp comprises aquartzenvelope 10 which may be formed from some hard glass. A pair ofcylindrical electrodes 12 and 14 are fixed in the envelope and extendthrough the envelope. The electrodes 12 and '14 are connected throughthe leads '16 and 1% to a source of electric current 19.

The envelope 1% is tilled with a rare gas, a vapor, or combinationthereof, preferably xenon gas plus a drop of mercury. The mercury isvaporized on heating by the discharge. In some cases it may be desirableto use mercury alone and the vapor pressure will therefore be relativelylow.

In the preferred embodiment of the invention, the pair of electrodes 12and 14 are tungsten and are connected as an anode and a cathoderespectively. The electrodes =12 and 14 are arranged in spaced coaxialalignment, wherein, the spacing is relatively small compared with thediameter of the envelope 10. The electrode 12 has a concave curvedsurface of revolution 20 formed therein and a raised portion 22 whichextends upwardly into the area surrounded by the curved surface 20. Thecurved surface 29 is generally similar to the frustum of a sphere. Theportion 22 resembles a plateau which extends upwardly into the frustum.

The curved surface 20 is spherical with an origin at the center of thebright portion of an electric arc. The bright portion of the electricarc is approximately at the tip of the cathode. Generally speaking theorigin of the genetrix is placed at about the distance from the apex ofthe electrode l4 to the portion 22 of the electrode '12. According tothe preferred embodiment the origin is approximately /2 to 2 mm. fromthe apex of the electrode l4; and the radius of curvature isapproximately 6 mm. Generally the radius of curvature will beapproximately 1.3 times the electrode spacing.

The spherical portion 29 reflects a portion of light through the highintensity portion of the arc, and through a preselected portion of thequartz envelope to thereby increase the intensity in one quadrant of thelamp.

A second embodiment of the invention is illustrated by FIG. 3, wherein areflecting portion 36) is formed in an electrode 32. The electrode 32 isconnected as a cathode by any suitable means 33. The reflecting portion30 comand has a genetrix prises a spherical surface of revolution withan origin in axial alignment with an axis of the electrode 32. A coneshaped inner portion 34 extends upwardly through the spherical portion30 and extends beyond the portion 30. The spherical surface has anorigin above the apex of the cone shaped portion 34 which lies on theaxis of the electrode 32. The origin is approximately the distance fromthe apex of the cone shaped portion 3 4 to a conventional anode 36.

FIG. 4 illustrates the polar distribution of a lamp according to thefirst embodiment of the invention compared with a conventional compactsource lamp. The polar taxis O180 corresponds to the axis of theelectrodes. For comparison purposes the lamps are arranged with theircathode; at the top and correspond to the 0 portion. The center of theelectric arc corresponds to the intersection of the 0, 90 and 180 axis.The intensity of the lamp is measured at various points along a meridiancurve i.e. at various points equidistant from the center of the lamp inorder to indicate the illumination intensity along the polarcoordinates. The heavy solid :line indicates the polar distribution of alamp according to the first embodiment of the invention; and, therelatively light dashed line indicates the distribution of aconventional lamp. These curves are relative polar distribution curvesof the respective lamp and do not indicate the total intensity output.

What is claimed is:

1. An electric discharge lamp comprising a transmis sive envelope, arare gas filling said envelope, and a pair of axially aligned solidelectrodes fixed within said envelope, and means electrically connectingsaid electrodes with the outside of said envelope, a first one of saidelectrodes including an integral reflector comprising a concavespherical surface generally similar to the frusturn of a sphere and acentral raised portion extending upwardly into an area defined by saidspherical surface, and a second electrode of said pair of electrodesspaced from said first electrode and having a convex conical shape.

2. An electric discharge lamp comprising a transmissive envelope, a raregas filling said envelope, and a pair of axially aligned solidelectrodes fixed within said envelope in coaxial alignment, means forelectrically connecting said electrodes with the outside of saidenvelope, and a first one of said electrodes including an integralreflector comprising a concave spherical surface of revolution generallysimilar to a frusturn of a sphere and a central raised portion extendingupwardly into an area defined by said spherical surface of revolution, asecond electrode of said pair of electrodes having a convex conicalshape, said surface of revolution having an origin lying on the axis ofsaid electrodes and between said pair of electrodes at approximately thedistmce from an apex of said cone shaped electrode to the raise-dportion of said first electrode.

3. An electric discharge lamp comprising a quartz envelope, a rare gasand a quantity of mercury filling said envelope, a pair of solidelectrodes fixed within said envelope in coaxial spaced alignment andmeans for electrically connecting said electrodes to the outside of saidenvelope, the spacing between said electrodes being between /2 mm. and15 mm., a first one of said electrodes including a concave sphericalreflecting portion generally similar to a frustum of a sphere and acentral raised portion including a relatively fiat top portion extendingupwardly into an area defined by said spherical portion, a second one ofsaid electrodes including a convex conical shape on one end thereof,said spherical reflecting portion having a radius of approximately 1.3times the electrode spacing and an origin on the axis of said electrodesand between said electrodes at approximately the distance from an apexof said cone shaped electrode to the raised portion of said firstelectrode.

References Cited in the file of this patent UNITED STATES PATENTS1,999,686 De Am-icis Apr. 30, 1935 2,459,516 Francis et al Jan. 18, 19492,887,603 Haidinger May 19, 1953 FOREIGN PATENTS 919,962 France Dec. 16,l9'4 6

1. AN ELECTRIC DISCHARGE LAMP COMPRISING A TRANSMISSIVE ENVELOPE, A RAREGAS FILLING SAID ENVELOPE, AND A PAIR OF AXIALLY ALIGNED SOLIDELECTRODES FIXED WITHIN SAID ENVELOPE, AND MEANS ELECTRICALLY CONNECTINGSAID ELECTRODES WITH THE OUTSIDE OF SAID ENVELOPE, A FIRST ONE OF SAIDELECTRODES INCLUDING AN INTEGRAL REFLECTOR COMPRISING A CONCAVESPHERICAL SURFACE GENERALLY SIMILAR TO THE FRUSTUM OF A SPHERE AND ACENTRAL RAISED PORTION EXTENDING UPWARDLY INTO AN AREA DEFINED BY SAIDSPHERICAL SURFACE, AND A SECOND ELECTRODE OF SAID PAIR OF ELECTRODESSPACED FROM SAID FIRST ELECTRODE AND HAVING A CONVEX CONICAL SHAPE.